This invention relates to a filtering member and more particularly to a cylindrical filtering member for use in an oil cleaner and the like.
There have been developed various kinds of filtering methods. These methods can be roughly classified into a surface filtering method and a bulk filtering method. In the surface filtering method impurities or admixtures contained in a liquid to be treated are collected or filtered out at a surface of a filtering member, i.e. a filtering material. When the impurities are deposited on the surface of a filtering member, resistance against a flow of the liquid passing therethrough is gradually increased and at last pores in the surface are completely clogged by the collected impurities, so that the filtering operation can no longer be effected. Therefore it is necessary to use a filtering member which has a very large surface area. This results in increase in size of a filter in which such a large filtering member is installed. On the contrary in the bulk filtering method impurities in a liquid are collected in a number of capillary spaces each having a sufficient length viewed in a direction of a liquid flow as well as in a number of small spaces in the bulk of the filtering member. In this manner the impurities are collected in the bulk of filtering material in a three dimensional manner and thus a great number of impurities can be filtered out by means of a filtering member having a relatively small volume.
By means of the bulk filtering method in order to remove impurities of different sizes in a liquid such as a working oil which circulates in an oil pressure apparatus use may be made of a composite body of cellulose having continuous pores, a sintered body, foamed synthetic resin, etc. However since the size of the impurities varies over a very wide range it is quite difficult to select suitable filtering material. If fine impurities are to be collected, a filtering member might be clogged within a relatively short time, and thus in order to prolong a useful time the filtering member has to be formed in such a manner that only impurities having large size can be collected. Then small impurities might remain in the oil without being filtered out and might damage the oil pressure apparatus. An ideal filtering member for the bulk filtering should have pores of large size at an upstream side and should have pores of small size at a downstream side viewed in a direction of a liquid flow. Then large impurities are collected at the upstream portion and small impurities at the downstream portion of the filtering member. In this manner the impurities of various sizes can be effectively collected in the bulk of the filtering member. Therefore the deterioration of the filtering material occurs equally or evenly at various portions thereof and thus the filtering efficiency is materially prolonged. From this view point a plurality of filtering layers having different porosities are stuck one upon the other to form a laminated filtering member. However such a filtering member is quite complicated in construction and can be manufactured only with difficulty. Therefore the cost of such a filtering member becomes rather high.